1. Field of the Invention
This invention relates to multicarrier transmission schemes such as Discrete Multitone (DMT) and Orthogonal Frequency Division Multiplexing (OFDM) as well as to transmitter, receiver, and/or transceiver apparatus and methods.
2. Discussion of the Related Art
Multicarrier transmission schemes such as DMT and OFDM are becoming widely used in such areas as Digital Subscriber Line (xDSL), Digital Audio Broadcasting (DAB), Digital Video Broadcasting (e.g. DVB-T, DVB-S, DVB-H) and wireless Local Area Networks. Multicarrier transmission schemes have many advantages, including high spectrum efficiency, resistance to interferers and noise and resistance against multipath interference. One less desirable property of multicarrier transmission schemes is that the transmitted signal has a very high crest factor. The crest factor of a signal is defined as the ratio of the peak amplitude of the signal to the Root Mean Square (RMS) value of the signal. This is also known as Peak to Average Ratio (PAR). The high crest factor poses challenges for the analog front-end design of a multicarrier transmitter and increases the power consumption of the front-end, largely due to the needs of the power amplifier.
One known solution to reduce crest factor is to clip the signal, before transmission. Indeed, the digital to analog converter (DAC) in a multicarrier transmitter may inherently clip the signal if the signal exceeds the range of the DAC. Clipping has a disadvantage of distorting the signal, which can result in erroneously decoded data symbols and hence bit errors at a receiver. In frequency division multiplexed systems, such as digital subscriber line (xDSL), where signals for the upstream and downstream paths are frequency multiplexed, clipping in one path can cause errors in the other path. Another known solution to limit crest factor is to use pulse shaping techniques, which can reduce the harsh effects of clipping. However, if pulse-shaping is used there is a need for extra filtering in the analog front-end of receivers to remove the out-of-band distortion which pulse-shaping incurs and this also increases the cost of transmitters. A further known method to reduce crest factor reserves certain ones of the carriers (tones) to create symbols with lower crest factor but this has the disadvantage of permanently decreasing the data rate. A further known method reserves some bits for messaging between transmitter and receiver in order to properly decode the modified symbols (with lower crest factor). This also has a disadvantage of a permanently decreased data rate and requires compatible receivers.
U.S. Pat. No. 6,757,299 describes peak power to average power reduction in multicarrier communication systems. A subcarrier symbol is identified which has maximum effect on a peak in a frame and the symbol on this carrier is modified to reduce the peak. This method concentrates solely on reducing peak size, which could incur a high or unacceptable bit error rate. Once a peak has been detected, the method requires a new symbol to be computed and then transformed from the frequency domain to the time-domain.
The present invention seeks to provide an improved way of reducing the crest factor in a multicarrier transmission scheme.